The influence of different drying conditions on the compressive strength and strength development rates of high strength concrete up to an age of 28 days was evaluated. Two HSC mixes with and without silica fume addition were used to cast cubes of 10 cm size. The cubes were stored in different drying conditions until the age of testing at 3, 7, 28 days

Kohoutková, A.

Safan, M.

Acta Polytechnica

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionFor many years, concrete with compressive strengthhigher than 400 kg/cm2 (40 MPa) was only available in fewlocations. In recent years, the applications of high strengthconcrete (HSC) have increased as a result of recent develop-ment in material technology and in the demand for HSC.The construction of the world tallest concrete buildings, pre-stressed concrete bridges, and other special structures wouldnot have been possible without the development of HSC [1].High strength concrete offers greater compressive strengthper unit cost, weight, and volume associated with improvedmechanical properties such as modulus of elasticity and ten-sile strength. Besides the satisfactory appearance of finishedsurfaces and possible reduction in the size of structural mem-bers, HSC has remarkably improved durability characteristicsand thus, serviceability and maintenance problems are ex-pected to decrease. Because of its superior characteristics,high strength concrete can be applied in a wide range ofapplications. It can be recommended in nuclear vessels due toits low permeability, can be applied in aggressive environ-ments due to its high durability, and can also be a convenientdesign alternative in high rise buildings, prestressed bridgegirders, and composite structures. Curing is a critical processfor the production of high strength concrete, taking intoaccount the low water-cement ratios usually applied in con-crete mix design. Mechanical characteristics and their de-velopment rate are highly influenced by curing conditions. Inthe current research, crushed dolomite was used in two highstrength concrete mixes with 0.0 and 10 percent silica fumeaddition to evaluate the influence of five different dryingconditions on high strength concrete compressive strengthevaluated at 3, 7, 28 days.2 Methods of HSC ProductionHigh strength concrete can be produced by conventionaland special techniques for production. In the former, thefollowing considerations are usually adopted [1], [2]:• Unless high early strength is needed, as in prestressedconcrete, the most common types of portland cement(ASTM types I, II, III) can all be used to produce highstrength concrete.• Water reducers are commonly used to reduce the water-ce-ment ratio as the most significant single factor affectingconcrete strength.• Crushed coarse aggregates with lower nominal maximumsize are known to produce higher strength.• Efficient compaction and curing are essential.Compared to normal concrete mixtures, high strengthconcrete mix proportioning is a more critical process andthus, many trail batches may be required to provide data thatwill enable the designer to identify the optimum mix pro-portions. On the other hand, several exotic techniques can beapplied to produce HSC. These methods include [3]:• Modification with polymers.• Use of active or artificial aggregate such as portland ce-ment clinkers to enhance the paste-aggregate bond.• Introducing a tri-axial stress state by using finely dividedreinforcements such as wires, nails, steel wool high densityfibers, etc.• Increasing compaction by pressure.• Autoclave curing.3 Admixtures in HSCNearly all high strength concrete mixes have containedchemical admixtures. Conventional water reducers, retarders,and high range water reducers (HRWR) or superplasticizersare typical examples. Changes in the dosage and combinationof these admixtures affect both the plastic and the hardenedproperties of concrete [1]. Also, finely divided minerals orpozzolanic admixtures such as fly ash and silica fume havebeen used to achieve higher strengths and better durability.Silica fume is a by-product material resulting from the reduc-tion of high purity quartz with coal in electrical arc furnacesduring the production process of ferro-silicon and siliconmetal [4]. It can be obtained in a loose, condensed, andslurry form. Because of its extreme fineness (specific area of20 m2/g), silica fume has a high water demand and, like otherpozzolanic materials, behaves more efficiently in concretemixes having higher water-cement ratios. The action of silicafume is twofold; it reacts with free lime in cement to forma new cementitious compound, calcium silicate hydrate, andenhances the paste-aggregate bond [5], [6]. Silica fume canbe applied either as an addition or as a partial replacementof cement. In the first approach, typical silica fume ratiosbetween 5 to 15 percent by weight of cement are usuallyadded. The increased water demand associated with silicafume addition requires the use of a HRWR to maintain24 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No. 3/2001Influence of Different Drying Conditionson High Strength Concrete CompressiveStrengthM. Safan, A. KohoutkováThe influence of different drying conditions on the compressive strength and strength development rates of high strength concrete up to anage of 28 days was evaluated. Two HSC mixes with and without silica fume addition were used to cast cubes of 10 cm size. The cubes werestored in different drying conditions until the age of testing at 3, 7, 28 days.Keywords: high strength concrete, curing conditions, silica fume, high range water reducers.the water-cementitious ratio required to achieve the desiredstrength [7]. However, if more water is added to compensatethe loss in slump as an alternative to the use of a HRWR, therewill still be a marked increase in the compressive strength [8].4 Curing conditionsCuring is the process of maintaining satisfactory moisturecontent and a favourable temperature in concrete duringthe hydration of the cementitious material so that desiredproperties of concrete can develop. The potential strengthand durability of concrete will not fully develop, unless it isproperly cured for an adequate period prior to being placedin service. At water-cement ratios below 0.4, the ultimatedegree of hydration is significantly reduced if free water is notprovided. Water curing will allow more efficient, although notcomplete hydration of the cement [9]. Kliger [10] reportedthat for lower water-cement ratios it is more advantageous tosupply additional water during curing. For concrete mixeswith water-cement ratios of 0.29, the 28 day compressivestrength of specimens cast with saturated aggregates andcured by ponding water on top was 59 to 69 kg/cm2 greaterthan that of comparable specimens cast with dry aggregatesand cured under damp burlap. Also, it was noted that whileearly strength is increased by elevated temperature duringmixing and curing, later strengths are reduced by such tem-perature. However, Pfieffer [11] has demonstrated that laterstrengths may have only minor reductions if heat is appliedafter setting.5 Experimental workThe current research was conducted in the strength ofmaterials laboratory, Menofia University, Egypt. Crushed do-lomite was used in two mixes with a HRWR and 0.0 % and10.0 % silica fume addition to evaluate the effect of differentdrying conditions on HSC compressive strength. The dryingconditions are described in Table 1 indicating the averageof temperature and relative humidity in each case. A totalnumber of 90 cubes (100 mm, side length) were been testedat different ages (3, 7, and 28 days) to explore the strengthdevelopment rates up to an age of 28 days. Concrete mixproportions of mix M1 (0.0 % S. F.) and mix M2 (10.0 % S. F.)are reported in Table 2.6 Materials• Cement: ordinary portland cement was used. Compressivestrength of mortar cubes (70 × 70 × 70 mm) measured at3 and 7 days was 180 and 255 kg/cm2, respectively.• Aggregates: natural siliceous sand was used. The deliveredsand had a fineness modulus of 2.51. Half of the sandweight was sieved using sieve No. 30. Sand retained onthis sieve was blended with the rest of the sand to obtaina coarser sand with a fineness modulus of 2.83 satisfy-ing the material recommendations in [13], [14] to achievebetter workability and compressive strength as the finenessmodulus approaches a value of 3.0. Crushed dolomite(specific gravity = 2.59 and volume weight = 1.53 t/m3)with a maximum nominal size of 0.5 in. (12.5 mm) was usedas coarse aggregate. The particles were irregular, angularin shape with a relatively high percentage of flat andelongated particles, and had a granular-porous texture.Grading curves for fine and coarse graded aggregates areshown in Figures 1, 2 along with the limits according toASTM C33-82 [12].• Water: tap water was used for mixing and curing.• Chemical admixtures: one type of admixture with the tradename Addecrete-BDF was used in the two mixes. Theadmixture produced by Modern Construction Chemicals,©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 25Acta Polytechnica Vol. 41  No. 3/2001Case Drying Condition Relative Humidity [%] Temperature [°C]1 Water immersion 100 262 Laboratory atmosphere 79.0  2 29.5  13 Lab. Atmosphere + spraying * Similar to case (2)4 Direct sun + spraying * Similar to case (5)5Direct sun:• morning• noon• night49.0  320.0  582.0  529.5  239.0  124.5  1* twice a dayTable 1: Drying conditions and associated average measured temperature & relative humidity in surrounding environmentMix Concrete Mix Proportions [kg/m3] Density[kg/m3]C. A./F. A. S/C[%]W/(C+S)[%]A/(C+S)[%]Slump[mm]C W F. A. C. A. S AM1 450 146.9 728 1092 0.0 1338 2430 1.5 0.0 32.6 2.97 70M2 450 133.7 720 1097 45 13.5 2441 1.5 10 27 2.73 55C = Cement W = Water F. A. = Fine AggregateC. A. = Coarse Aggregate S = Silica fume A = Admixture (HRWR)Table 2: Concrete Mix Proportions [ N. M. S of C. A. (crushed dolomite) =1/2 in. and sand F. M.= 2.83]Cairo, Egypt is classified as a high range water reducermeeting the requirements of ASTM C494 (type A, F).The admixture is a brown liquid with 1.165 kg/l density at20 °C.• Silica fume: silica fume provided by the Free Silicon Com-pany, Cairo, Egypt was used in a powder form witha light gray color giving a black slurry when mixed withwater. The product had a specific gravity of 2.2, bulkdensity = 280 kg/m3, and specific surface area = 17 m2/g.• Mixing procedure:A 50 liter capacity mixer was used to mix batchesof 20 liters needed to cast fifteen cubes. The mixer wascharged with the drum at rest using the following se-quence: half of the coarse graded aggregate, sand, andcement and silica fume mixed together. After charging,water was added gradually during mixing that continuedfor 2 minutes, the HRWR was added and mixing continuedfor 3 minutes to ensure adequate mixing. The concrete wascharged from the mixer bowl and the slump test was thenperformed according to ASTM C143-78 [13]. The concretespecimens were removed from the steel molds 24 hours af-ter adding the mixing water and stored according to thedifferent drying schemes until the age of testing.7 ResultsThe test results at 3, 7, and 28 days are presented inFigures 3, 5 for mix M1 and in Figures 4, 6 for mix M2. Eachresult represents the average of three tested cubes.Test results for mix M1 (0.0 % S. F.): Figure 3 shows that the28-day compressive strength for case 1 was higher than theother cases. The strength reduction increased with increaseddryness of the curing environment, reaching a maximum of15 % in case (5). However, this was not the case at early ages.At an age of 3 days, case (1) did not provide the most favour-able curing condition, where strength increments between16 and 21 percent were recorded for the other cases. With thepassage of time, the loss of water due to evaporation abatedthe hydration process resulting in the strength reductionobserved at 7 and 28 days.26 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No. 3/2001020406080100120No. 100 No. 50 No. 30 No. 18 No. 8 No. 4 3/8 in.SIEVE SIZEASTM C33ASTM C33used F. A.PERCENTAGEPASSINGFig. 1: Grading of the used sand (F. M.  2.83)01002003004005006007003 7 28AGE [days]case (1)case (2)case (3)case (4)case (5)Fig. 3: Compressive strength for different drying conditions at different ages (silica fume content = 0.0)020406080100120No. 18 No. 8 No. 4 3/8 in. 1/2 inSIEVE SIZEASTM C33ASTM C33used C. A.PERCENTAGEPASSINGFig. 2: Grading of crushed dolomite (M. N. S  1/2 in)Test results for mix M2 (10.0 % S. F.): Figure 4 shows that dif-ferent drying conditions have a remarkable effect on strengthespecially at 3 days age. At this age, a gradual increase instrength reaching a maximum of 35 % for case (5) wasrecorded. In case (5) a maximum temperature of 46 °C wasmeasured at the specimen surface. This relatively high tem-perature simulated the cement hydration process and thepozzolanic reaction of silica fume, resulting in a remarkablestrength increase. This phenomenon was also observed atlater ages of 7 and 28 days. Because of its low permeability,silica fume concrete specimens kept a moisture level sufficientfor the hydration process to proceed efficiently even after 28days in a dry environment. Figures 5, 6 show the compressivestrength development at different ages, indicating that case 1provided better rates of strength gain than any other curingconditions.8 ConclusionsConcrete specimens (cubes 100 × 100 × 100 mm) werecured in different drying conditions (water immersion, labo-ratory atmosphere, laboratory atmosphere and spraying, di-rect sun and spraying, and direct sun). The specimens weretested at 3, 7, and 28 days. The trend according to which thecompressive strength was affected by drying condition variedat different ages. It is recommended that testing should©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 27Acta Polytechnica Vol. 41  No. 3/2001010020030040050060070080090010003 7 28case (1)case (2)case (3)case (4)case (5)AGE [days]Fig. 4: Compressive strength for different drying conditions at different ages (silica fume content  10 %)3003504004505005506006500 5 10 15 20 25 30case (1)case(2)case (3)case (4)case (5)Silica fume content = 0.0AGE [days]Fig. 5: Compressive strength for 100 × 100 × 100 mm concrete specimens for different curing conditionsextend in time and for different specimen sizes to providebroader understanding of the behaviour trend. However, thefollowing conclusions could be drawn:• High strength concrete specimens with 10 percent silicafume addition were more sensitive to drying conditionsthan those with zero silica fume content.• The compressive strength of silica fume concrete speci-mens increased as the curing environment temperatureincreased. This effect was more obvious at early ages anddecreased with the passage of time.• Because of its low permeability, silica fume concrete kepta moisture content level that was sufficient for the hydra-tion process to proceed efficiently even after 28 days ina dry environment.• Continuously water-cured concrete specimens provideda better rate of strength development compared to othercuring conditions.References[1] State-of-the art report on high strength concrete. ACI Com-mittee 363 R–1984[2] Macinnis, C., Thomson, D. V.: Special techniques forproducing high strength concrete. ACI Journal, Vol. 67,No. 12/1970, pp. 996–1002[3] Duriez, M.: Methods of achieving high strength concrete. ACIJournal, Vol. 64, No. 1/1967, pp. 45–48[4] High strength concrete. Manual of Concrete Materials –Aggregates, National Crushed Stone association,Washington, D. C., January 1975, p. 16[5] Howard, N. L., Leatham, D. M.: The production and deliv-ery of high strength concrete. Concrete International, April1989, pp. 26–30[6] Rosenberg, A. M., Gaidis, J. M.: A new mineral admixturefor high strength concrete. Concrete International, April1989, pp. 31–36[7] Luciano, J. J., Nami, C. K.: A novel approach to developinghigh strength concrete. Concrete International, May 1991,pp. 25–29[8] Malhotra, V. M., Carette, G. G.: Silica fume concrete prop-erties: applications and limitations. Concrete International,May 1983, pp. 40–46[9] Neville, A. M.: Properties of concrete. 3rd Edition, PitmanPublishing Limited, London, 1981, pp. 779[10] Kliger, P.: Early high strength concrete for prestressing.World Conference on Prestressed Concrete, San Fran-cisco, 1957, pp. A5 (1–14)[11] Pfieffer, D. W., Landgreen, J. R.: Energy efficient acceler-ated curing of concrete: a laboratory study for plant-producedprestressed concrete. Technical Report No. 1, PrestressedConcrete Institute, Chicago, December 1981[12] ASTM C33 – 82: Specification for concrete aggregates.[13] ASTM C143 – 78: Specification for ordinary portland cement.Eng. Mohamed Safan, MScphone: +420 2 2435 4620e-mail: msafan@beton.fsv.cvut.czIng. Alena Kohoutková, CSc.phone: +420 2 2435 3740e-mail: akohout@fsv.cvut.czDept. of Concrete Structures & BridgesCzech Technical University in PragueFaculty of Civil EngineeringThákurova 7, 166 29 Praha 6, Czech Republic28 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No. 3/20014505005506006507007508008509009500 5 10 15 20 25 30case (1)case (2)case (3)case (4)case (5)Silica fume content = 10.0AGE [days]Fig. 6: Compressive strength for 100 × 100 × 100 mm concrete specimens for different curing conditions

Influence of Different Drying Conditions on High Strength Concrete Compressive Strength

M. Safan

A. Kohoutková

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Influence of Different Drying Conditions on High Strength Concrete Compressive Strength

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